If you’ve recently updated to Mac OS X 10.7 or 10.8, you may have received the message:

“To open this Web Start application, you need to download
the Java Runtime Environment.”

That’s because Apple Java was removed during the latest update. So when you receive this message, click “More Info“, and you will be taken to the java.com website for Version 7 at http://www.java.com/en/download/help/mac_install.xml.Note: Once you’ve installed Java, you may need to reload or quit your browser in order to enable Java in your browser.

If you’re running Mountain Lion OS X 10.8, Gatekeeper (the new program introduced in OS X 10.8 designed to prevent potentially malicious apps from launching) may block the launch of the PhET sim. To override this, either modify Gatekeeper’s default settings or explicitly right/control-click the application and choose Open.